There’s a hopefully unrelated concept called purple urine bag syndrome I have seen. Not completely understood either but this paper thinks due to a combination

There’s a hopefully unrelated concept called purple urine bag syndrome I have seen. Not completely understood either but this paper thinks due to a combination of dietary tryptophan breakdown from constipation and colonic E coli load, urinary bacteria, and reaction with the plastic tubing of the catheter and bag. https://pmc.ncbi.nlm.nih.gov/articles/PMC3894016/ https://pmc.ncbi.nlm.nih.gov/articles/PMC3894016/